1. Field of the Invention
The present application relates to an apparatus for controlling an air-fuel ratio of an internal-combustion engine.
2. Discussion of the Background
An air-fuel ratio control apparatus having the function of determining an imbalance fault on the basis of a signal output from an air-fuel ratio sensor in an engine exhaust system is described in Japanese Unexamined Patent Application Publication No. 2011-144754. That apparatus executes air-fuel ratio fluctuation control of causing an air-fuel ratio at a predetermined frequency during an engine operation and determines an imbalance fault using a determination parameter obtained by dividing the strength of a 0.5th-order frequency component contained in a signal output from the air-fuel ratio sensor during the execution of the control by the strength of a predetermined frequency component. The 0.5th-order frequency component is the ½ frequency component of the frequency corresponding to a rotation speed of the engine. When an imbalance fault occurs, the strength of the 0.5th-order frequency component increases, and the value of the determination parameter increases with an increase in the degree of the imbalance. Accordingly, an imbalance fault can be determined by comparing the value of the determination parameter with a predetermined threshold.
With the technique described in Japanese Unexamined Patent Application Publication No. 2011-144754, when the response characteristic (frequency characteristic) of the air-fuel ratio sensor degrades and the strength of the 0.5th-order frequency component decreases, the strength of the predetermined frequency component also decreases. Thus, the use of the determination parameter, which is a ratio, in both enables the determination accuracy to be maintained high.